Wing skins for aircraft are typically machined from metal plate stock that is essentially flat on both sides. In accordance with one known technique for machining a wing skin, a plate is held down on a mill bed by the use of vacuum exerted on an under surface of the plate. The upper surface of the plate is then machined to the desired contour. The first side machined is generally the aerodynamic surface, also known as the "outside mold line" or OML. The majority of the OML surface is smooth, but at the inboard end of the wing skin there typically are protruding features such as padups, steps, or taper planes serving to enable the wing skin to be attached to the fuselage or other structure.
After the OML surface is machined, the wing skin is turned over on the mill bed so that the other surface of the plate can be machined to form the "inside mold line" or IML. The protruding features at the inboard end of the wing skin are accommodated in pockets or depressed regions of a plate-shaped metallic adapter tool that fits into a well area defined in the mill bed. This adapter tool enables the wing skin to fit snugly against the seal that engages the wing skin for vacuuming the wing skin down onto the mill bed so that the IML can be machined.
Each aircraft model has unique wing skin configurations with unique protruding features, and hence, whenever it is desired to machine a new wing skin configuration, the existing adapter tool must be removed from the well area of the mill bed and a new adapter tool having the appropriate configuration for the new wing skin must be installed in the well area. Each such adapter tool typically can be 60 inches wide, 80 inches long, and 1.125 inches thick, and can weigh 600 pounds. Accordingly, it will be appreciated that the adapter tools cannot be handled manually, but must be moved through the use of heavy equipment such as cranes. It can take two hours for removing an adapter tool and installing a new adapter tool in the mill bed. Every time a new wing skin configuration is to be machined, the adapter tool must be removed and replaced with a different one. Thus, the significant time required for changing the heavy adapter tools introduces considerable inefficiencies in the manufacturing process. Furthermore, a significant capital expenditure is required where a substantial number of different wing skin configurations must be machined, because each wing skin configuration requires its own adapter tool, and each tool can be quite expensive.